During the past year we have utilized length polymorphisms in simple sequence repeats or "microsatellites" to complete the first allelotype analyses of murine tumors. Lung and mammary adenocarcinomas of B6C3F1 mice induced by butadiene and methylene chloride were examined. Frequent losses of heterozygosity were observed on chromosome 4 in the lung tumors. The pattern of allelic losses was consistent with a region previously implicated as the site of a tumor suppressor gene by cell hybrid studies. In addition homozygous deletions have been reported in a homologous region of human chromosome 9p for a variety of human tumors including lung carcinomas. The most common allelic losses in mammary tumors were detected on chromosomes 11 and 14 near the p53 and retinoblastoma tumor suppressor genes, respectively. Mutations in the p53 gene were characterized in these tumors by immunological techniques, single strand conformation polymorphism analysis, and direct DNA sequencing. We have also demonstrated the power of microsatellite PCR for human allele loss studies. Losses of heterozygosity in sporadic human breast and ovarian cancers were mapped in detail using a series of microsatellites in the proximal region of 17q near the BRCA1 gene. This gene confers a strong familial predisposition to breast and ovarian cancer. Allele losses were observed in more than 70% of the tumors examined providing strong support for the hypothesis that inactivation of the BRCA1 gene plays a critical role, not only in rare breast and ovarian cancer families, but in very common sporadic tumors as well. As part of this work we developed two new highly informative polymorphisms that lie within the p53 and thyroid hormone receptor alpha (THRA1) genes. Our allele loss studies implicated THRA1 as an excellent candidate for the BRCA1 gene. However, mutation analyses of sporadic breast tumors and German breast cancer families excluded THRA1 as the BRCA1 gene. In collaboration with Dr. Skolnick's group, our current studies are directed at the isolation of BRCA1 by positional cloning techniques. This includes examining additional tumors for allele losses and generating a physical map of a 500 kb region of 17q.